Can James McAvoy and Michael Fassbender make us believe they're playing the same characters as Patrick Stewart and Sir Ian McKellen, in X-Men: First Class? We have high hopes. But recasting iconic characters doesn't always work out that great.

Sometimes you swap out an actor, and believability — and character growth — go out the window. Sometimes a recasting just plain does not work. Here are the 10 worst recastings of science fiction's great characters.

Jeffrey Weissman as George McFly in Back To The Future II and III. According to IFC, Crispin Glover refused to come back to the BTTF films as George McFly, which caused a problem for the two sequels. So director Robert Zemeckis and producer Bob Gale decided to replace Glover with actor Jeffrey Weissman, and disguise the substiution by using clips of Glover interspersed with new footage of Weisman disguised with prosthetics, wigs... and even hanging him upside down at one point.

Chloë Annett as Kristine Kochanski on Red Dwarf. As the Onion AV Club puts it, "Kristine Kochanski, the love interest in the British cult science-fiction comedy Red Dwarf, underwent a curious arc, with a downgrade in personnel attending an upgrade in character." She started out being played by the terrific Clare Grogan, as a woman Lister had fancied and whose holographic image he lusted after. But eventually the writers decided to retcon her as having been Lister's actual girlfriend, and introduced her counterpart from an alternate universe as a character on the show — only to recast the part as the somewhat less interesting, if more model-looking, Annett.

Mary Alice as The Oracle in The Matrix Revolutions. This one is mostly annoying for how much attention they drew to the change, and the silly rationalization the Wachowskis cooked up to explain it. When the original Oracle, Gloria Foster, died of a complications from diabetes, the Wachowskis replaced her with Alice, who didn't quite have the same presence — but also, the film tried to make the change of actor a plot point, saying that the Oracle was re-shelled because of an incident within the Matrix and this proved that fate and free will are chaos and... where was I?

Halle Berry as Catwoman Catwoman is one of those characters who's been played by a ton of actors, including three Catwomen in the original 1960s Batman series and Michelle Pfeiffer in Batman Returns. But Halle Berry's incarnation, with the CG-animated ass and the horrendous scenes of trying to get her neighbors to turn their damn music down, was the worst reimagining of a character possible. At least Berry had the good grace to accept her Razzie Award for Worst Actress in person.

John Davey as Captain Marvel in Shazam! This is another one which the Onion AV Club mentioned. Check out the difference between the two versions of Shazam!'s opening credits here. In the first one, Captain Marvel looks at least somewhat like his comic-book self, albeit with huge mutton-chop sideburns and exploding 1970s hair. But in the second version, the show's producers have replaced the original Captain Marvel, Jackson Bostwick, in a hurry with a new guy, John Davey, because Bostwick was injured on the job and missed work to get treatment. The new Captain Marvel can only be described, charitably, as slouchy. Shaz... ow!

Tim Pocock as Scott Summers in X-Men Origins: Wolverine. It wasn't enough to kill Scott Summers off-screen in the third X-Men movie? Now they have to bring him back as a dumb teenage rebel, who's unable to deal with a bossy teacher and then gets caught way too easily by Sabretooth.

Everyone as Davros, in Doctor Who. This is really a case of an actor who defined a particular role so well, that nobody else could just pick up and carry on. Michael Wisher was Davros — you only have to watch the archival video interview with the chain-smoking Wisher on the "Genesis Of The Daleks" DVD to see how much of himself he put into the psychopathic scientist who created the evil Daleks. Unfortunately, nobody else who took on the role after Wisher was able to bring that same level of purring intellectual curiosity, mixed with batshit rage. It's hard to bring those extremes to the same character, and only Wisher — who'd also been a Dalek — could do it.

Everyone as Anakin Skywalker. This is almost too much of a cheap shot, but it has to be said. All of the actors who took over the role of Anakin Skywalker, the future Darth Vader, weren't up to the challenge of capturing Sebastian Shaw's performance in Return Of The Jedi. Shaw, a Shakespearean actor as well as a playwright and poet, brought a certain gravitas to the unmasked Anakin, not unlike James Earl Jones as Vader. But every actor who's tackled Anakin for the past decade or so, including Hayden Christensen and the animated version's Matt Lanter, has just lacked a certain... weightiness.

Robin Curtis as Saavik in Star Trek III and IV. We're still sad about this one, even 25 years later. Shelley Long should have stayed on Cheers, and Kirsty Alley should have been required to play Saavik in every Trek movie in the 1980s, without fail. Of all the let-downs about the third Trek movie, the replacement Saavik might have been the worst. She lacked all the wit and sparkle of the original, who was a great foil for Spock, showing how Spock had evolved from his more literal-minded youth and giving a taste of old-school Vulcan hidebound attitudes for Kirk to bounce off of. It would have been great to see Alley's Saavik dealing with suddenly teenage Spock, not to mention bearing Spock's baby in the fourth movie, as should have happened.

Everyone as John Connor. Edward Furlong wasn't exactly the best actor in Terminator 2, with his whole smirking "stand on one foot" routine. But his mixture of brattiness and raw pain, and his need for a father figure causing him to latch on to Arnie's tame death machine, was pretty memorable. And it's not hard to believe that John Connor, the leader of the resistance in the distant post-apocalyptic future, might have been a bit of a brat. It's a lot harder, though, to believe in any of the John Connors we've had since then, including Nicholas Stahl's neurotic twitchy version, Christian Bale's screaming tourettes version, and — sorry to say — Thomas Dekker's often way-too-pouty version. If only Furlong had been able to grow up with the character, that would have been really interesting to see.